The control of aircraft on the ground is a very complicated and high stakes task. Airfields must often provide control and guidance to aircraft while taxiing or standing both before takeoff and after landing. Safety and reliability are at a premium in the field of aircraft control on the ground. For this purpose, airfield lighting control systems have been developed to prevent incursions onto an active runway or taxiway, thereby reducing the possibility of collisions between aircraft, especially during conditions of low visibility.
Existing airport runways typically include a series of runway light fixtures secured along the centerline (embedded in the runway) and/or sidelines (embedded or raised) of an airport runway. Each light fixture includes one or more light sources, e.g., incandescent, fluorescent and/or light emitting diode (LED) lamps, which provide illumination and positional guidance to aircraft on the airport runway. Each light fixture includes one or more lamps that provide illumination and guidance to the aircraft. The runway light fixtures are powered by means of series connected isolating current transformers fed from a constant current regulator circuit located in an electrical vault near or beneath the runway. The constant current regulator typically is set at a current value that is selectable in steps from about 2.8 to 6.6 amperes RMS.
Control and monitoring of the runway lights are performed remotely at the control tower, and also at other maintenance locations. Thereby enabling airport personnel in the control tower to select individually the intensity (off being one intensity of zero) of the light illumination for each runway. In addition, lamps may be controlled to flashed on and off in various patterns.
Remote monitoring of the airfield lighting system is critical for safe operation. If an excessive number of runway lamps malfunction, especially if adjacent runway lamps malfunction, the lighting configuration of the runway may be adversely changed.
To effect remote control and monitoring of the airport lighting from the control tower, and other locations for maintenance and testing purposes, remote control and monitoring devices must be used in combination with the lighting fixtures, one control circuit for each lamp to be controlled, and enough monitoring points to insure that the runway lighting system is operating sufficiently to safely guide and control movements of aircraft on the runway(s). Typically, these control circuits are located close to or within each associated runway light fixture. Since electrical power conductors must be used to supply power to each runway light fixture, a popular and effective way to send control signals to and receive monitoring/status signals from each runway light fixture has been to impress power line carrier signals upon these electrical power conductors.
Using power line carrier control and monitoring has significant advantages as substantially less wire has to be run throughout the runway(s), and power line carrier signaling equipment is a mature technology that is relatively inexpensive to implement with today's electronic components. However, power line carrier signaling equipment suffers from some very basic and difficult to solve weaknesses (e.g., vulnerabilities) in systems operations and reliability. Power line carrier control and monitoring communications depends on the quality of the airfield power circuit wiring which is in a constant of degradation and will eventually develop faults. In addition, electrical noise/interference generated by power switching operations, lightning strikes and other man made and natural interference at the power line carrier signal frequencies degrades the operation/monitoring/speed of the airfield lighting control system. Also, if a power circuit is broken to a group of light fixtures that is used to feed the carrier signals to other groups of light fixtures, major runway lighting outages may occur. Operational speed of a power line carrier signaling system is also limited as the complexity of the runway lighting system increases. Electrical signal noise and interference will also reduce the effective speed of operation of the power line carrier signaling system.